This invention relates to a borosilicate, opal glass article composed of a phase-separated, opacified interior portion completely encased within, and integral with, a transparent, single-phase glass surface skin. The invention further contemplates a method of producing such a borosilicate, opal glass article by controlled heat extraction during glass forming.
An opal glass is an otherwise transparent glass having a light scattering material dispersed within its mass. The glass and the dispersed material have refractive indices which are sufficiently different from one another that light entering the glass is scattered rather than transmitted. Hence, the glass article appears translucent or opaque depending on the size and concentration of the dispersed material. In the absence of a glass colorant, the opacifying material normally imparts a white appearance to the glass. A glass colorant imparts its normal color to opal glass, although lightened or bleached by the white of the opacifying material.
It has been customary to produce an opal glass by adding to a glass batch a material, such as a fluoride or a phosphate, which is dissolved in the base glass during the melting operation, and then precipitates out in particle form as the glass cools (spontaneous opal), or on reheating (reheat opal). Initially, a soda lime type glass was employed as the base glass for the production of opal glasses, and most present day, commercial opal glasses are still of that nature. A soda lime glass is particularly desirable because of ease in melting and working, and potentially good chemical resistance. However, such glasses are generally characterized by a relatively high thermal coefficient of expansion. Hence, they have a low resistance to heat shock, unless physically strengthened as by air tempering.
The advent of borosilicate glasses in the 1910-1920 decade greatly extended the service areas for glassware. These glasses, because of their generally low thermal coefficient of expansion, proved particularly useful in service areas where heat was involved, such as laboratory glassware and baking ware.
It soon became desirable to opacify, that is, render opaque by dispersed material, the new borosilicate glasses. However, these new glasses not only melted at much higher temperatures than previous soda lime glasses, but had distinctly different chemical characteristics, including material solubility characteristics. In particular, the known opacifying materials did not dissolve and precipitate in the borosilicate glasses in the same manner as had been experienced in previous soda lime glasses.
Some of the problems involved in producing borosilicate opals, and efforts at the solution of these problems, are described in U.S. Pat. No. 3,275,492, granted Sept. 27, 1966 to J. L. Herbert, and the teachings of that patent are incorporated herein by reference. The patent teaches, in accordance with its invention, that certain divalent metal oxides, e.g., ZnO, BaO, and CaO, are opacifying agents capable of being precipitated in particle form in borosilicate glasses within a defined composition range. While dense opal glasses can be produced in accordance with the teachings of the Herbert patent, it has proven difficult to achieve a high degree of chemical durability in conjunction with high opal density.